Fluorocarbon containing plasma, such as CF.sub.4, CHF.sub.3, CH.sub.2 F.sub.2, and CH.sub.3 F, is commonly used in reactive ion etching (RIE) of polysilicon, silicon nitride and oxides. Unfortunately, it has been experienced that during a RIE process, undesired byproducts and residues, such as bubbles and spots, are formed due to high humidity and basic outgas. The residual byproducts are believed to react with acid-like sidewall polymers that are easily produced in fluorocarbon plasma.
The bubbles and spots can catastrophically interfere with the operation of a semiconductor device. For example, bubbles that are formed may cause incomplete etching of polysilicon layers, thus causing short circuits. The spots also generate similar problems during interconnect fabrication. Ultimately, the spots and bubbles reduce the yield of semiconductor processes.
FIG. 1 shows a cross-sectional view of a semiconductor wafer illustrating the bubbles formed by the conventional process. Using conventional techniques, a conductive layer 12 is formed on a substrate 10. The composition of conductive layer 12 is generally polysilicon. Then, a dielectric layer 14 is formed on the conductive layer 12. The dielectric layer 14 is typically an oxide layer. Patterning a photoresist layer 16 over the dielectric layer 14 allows controlled etching of the oxide layer 14. The main etching of the oxide layer 14 is performed by a fluorocarbon containing plasma etching process with an etching gas such as CF.sub.4, CHF.sub.3, CH.sub.2 F.sub.2, or CH.sub.3 F. The fluorocarbon containing plasma etching process is a reactive ion etching process. The steps above are often used in the fabrication of semiconductor devices, such as transistors and memory cells.
During the plasma etching process, the fluorine and carbon atoms of the plasma react with acid-like sidewall polymers so that bubbles 18 and spots are formed on the polysilicon layer 12. Because the bubbles 18 are formed over the polysilicon layer 12 as shown in FIG. 1, the bubbles 18 serves as a mask during later patterning of the polysilicon layer 12.
Thus, there exists a need for a reactive ion etching process using a fluorocarbon plasma that does not produce bubbles 18 or spots.